1. Field of the Invention
This invention relates to power conditioners for electrical distribution systems, particularly to active power conditioners, and especially to active power conditioners for mitigating zero-sequence currents in electrical distribution systems.
2. Description of the Prior Art
In a balanced three-phase, four-conductor electric power system, the algebraic sum of the source phase voltages, or source line-to-neutral voltages, is zero volts. Hence, in a WYE-connected system with balanced phase voltages and a balanced linear load, the neutral current, I.sub.n, is zero amperes, and the currents in the three phases are all equal in magnitude but differ by 120.degree. in Itime phase. A multi-phase load in which the impedance in one or more phases differs from those of other phases is said to be unbalanced. Many types of loads on current electric power.systems can be non-linear and unbalanced. Such loads can generate certain harmonics of currents and voltages in the phases of a three-phase system. Unbalanced loading can result in a fundamental (60 Hz) neutral current. Also, one consequence of non-linear loading of electric power systems is an increase of harmonic current in the neutral path.
In general, a current which appears in the neutral path at fundamental and harmonic frequencies as a result of non-linear loading or unbalanced loading may be termed a zero-sequence current. Zero-sequence currents in each of the phases can cause zero-sequence currents to appear in the neutral path. In power systems that supply a substantial non-linear load, neutral currents may exceed the rated ampacity of the neutral path, resulting in elevated temperatures and increased thermal losses in the neutral conductors and terminations. In extreme cases, excessive heating losses may damage the insulation of the neutral conductor and other conductors in the same conduit, causing the neutral conductor to fail as an open circuit, or damage materials in contact with the neutral.
A number of approaches have been applied to prevent or eliminate instances of neutral thermal overload. These include: (1) installing components with increased ampacity in the neutral path; (2) applying transformer arrangements to shunt zero-sequence currents out of the neutral path; and (3) using passive and active filters to shunt harmonic currents out of the neutral path.
Installing components with increased ampacity in the neutral path typically requires retrofitting existing electrical systems with either an upgraded neutral ampacity or a parallel neutral path. In either case, the necessary modifications could be prohibitively expensive.
Zero-sequence current shunt transformers typically act as current dividers, with the reduction of zero-sequence current being dependent on the ratio of the transformer zero-sequence impedance to the power system zero-sequence impedance. Such arrangements may filter out only about one-half of the zero-sequence neutral current, or less, which reduction may be insignificant relative to the system burden, and cost and complexity of the shunt transformer hardware.
Finally, passive and active filters can be difficult to implement and reliably operate, with difficult-to-predict behavior in practice. Such unpredictability can arise from transient ringing or resonant behavior at particular frequencies. With such filters, there are trade-offs between the filter's transient response and filter effectiveness. In addition, filters of this type typically have resistive components which consume real power.
There is a need, therefore, for a stable, active neutral current compensator which can nullify zero-sequence currents in the existing neutral line of a multi-phase power distribution system without consuming substantial amounts of real power to shunt fundamental and harmonic zero-sequence currents to ground.